I completely agree. Sadly, it's the nature of software development these days. We know that HGVC is switching over to a new system. Being in the software industry and being a part of these kinds of things in the past, it is common to have things "break" during a changeover like this. Behind the scenes, they are likely very different systems and the front end that you use works with both, at least partially. Making the necessary modifications for changing reservations could just be scheduled for a future update. Back to your mechanic's analogy - It's like you got your car back and it doesn't go into reverse and the mechanic says "come back in 6 months, we will see if it's ready to be fixed then"
Ok first of all, I'm the same way. I hate updates and put them off as long as possible. After a major version release, I try to wait at least one or two "dot" updates before I install the new version. The "dot" releases are usually fixes for big issues that they caused in the major update. As for why they released reservation changes broken, I'm guessing it's simply to do phased updates. They are converting over to a new system and they are working on what they consider the most important pieces first. They likely don't consider reservation changes important. What they will do is fix anything financial first. That's always the top priority. Quite honestly, what likely is happening is HGVC "went live" with what they consider "minimal required functionality", or slightly more. Likely it's a low percentage of reservations that get changed (according to their data), so changes are a lower priority and will be added back in later during the conversion. The reservation site is a toddler right now. It's still growing. Before they started this conversion, it was more like a 20-something year old. Generally pretty solid but you still had to shake your head at time.
There is a chance though, that this is the start of eliminating the ability to change reservations.